Fraxinus
by JoojooBrother
Summary: A couple of wacky Brits walk into a restaurant full of superheroes. There should be a joke in there somewhere, but honestly the Avengers are too exhausted from fighting off alien invasion. A letter is delivered and the universe needs saving in a much quieter way than nuking the mothership.
1. New Roots

**I found myself endlessly annoyed at the lack of thought that went into most crossovers for these two fantastic universes. With all the pseudo-science in both, a lot of fun could be had with it, so I decided to take a shot. I'm not a great writer, so this won't be very long. One or two chapters at the most. Allons-y!**

* * *

The moment Loki was successfully led away by Thor and some SHIELD agents to be chained, the Hulk's form shrunk down into a shivering Bruce Banner. Bruce, having dealt with two transformations in a single day, promptly collapsed against Tony, unconscious from exhaustion. The billionaire stumbled awkwardly in his depowered suit, nearly falling over himself.

Giving up the effort of standing while supporting heavy armour and another man's weight, Tony sunk down to sit comfortably in what he now was dubbing the 'Loki-crater' in the floor of the top level of Stark Tower. He was at least careful to set Bruce down in an area relatively void of glass, before activating the manual removal sequence on his suit.

Following their cue, the other Avengers seemed to collapse where they stood. Natasha slumped against a burnt settee along with Clint, and Steve gingerly sat down cross-legged, leaning against the bar.

"I think I'll keep the crater," Tony tried to joke, but it fell flat. It was actually pretty comfortable, as far as floor-craters went; and it was a decent place to drop bits of scorched armour in when removing the complex system of his suit by hand.

Steve glanced out over the edge of the tower, where he could hear sirens beginning to blare. Rescue efforts for people trapped under collapsed buildings and God knew what else were beginning.

The group of four gave collective inner groans. Steve was the first to get up, and it only took a glance between them for his meaning to be understood. As exhausted as the remaining Avengers were, they were still able-bodied enough to help rescue efforts. Tony eyed Bruce's unconscious form.

"Oi, Cap, put the Green Giant here on the couch before you go."

At Steve's look he added, "Hey, I'll come down to help too, just give me time to get out of this," he gestured to his armour, only partially dismantled, "and make a few calls. Do some major donations to rescue efforts," and give Rhodey an earful for not showing up, but that went unsaid.

Actually, best get on that now. "Jarvis," Tony tried, eyeing the general areas where he'd installed the speakers. Steve meanwhile, was carefully lifting the unconscious scientist.

Jarvis' voice system was damaged, but it managed, "_What-t are y-your wishes, sir_?"

"How's the elevator?"

There was a ding and the door opened. "_Op-perational, sir_," Jarvis put in. Tony felt mild annoyance, but at least something still worked right. The other three made their way inside, Steve giving Tony a final grim nod before the door closed.

Tony sighed before speaking to his AI, "Jarvis, see if you can contact Pepper again..."

They'd been helping at different points in the city when the army finally rolled in. Tony, Natasha, and Clint seemed content to leave it to them, but it had taken several civilian volunteers, the NYPD, and the temptation of a free meal courtesy of the local East-Indian joint before Steve finally stepped down.

Thor had returned to Stark Tower and picked up an exhausted Bruce before meeting them there. They had shoved two of the three remaining tables together and were just starting in on their ordered shawarma when the door opened to a young couple, unusually exuberant for the chaotic environment.

"_Brilliant_ business, still running after all that," the tall man with spiky brown hair said with a grin, looking around. If the blonde woman's union jack shirt wasn't an indicator, his distinct English accent certainly betrayed their foreign origins.

The blonde gave the group a lookover, sharing an excited look with her companion. They took their seats on the remaining table, ordering a lamb shawarma and two samosas. They kept sending sidelong glances to the group as if waiting for something. The blonde made to say something, but the male shushed her and they giggled.

"Know something we don't?" Tony couldn't resist asking.

"Yes, actually, quite a bit," the man said easily, "Just waiting for you to ask. I'm the Doctor."

"Rose Tyler," the girl added, "and we really need to think of a new name for you, Doctor."

"Hm? Oh yes, well, I suppose," said the Doctor, "I'm going to have to go by John Smith now, won't I?"

"Not John Smith!" Rose exclaimed, "You get a chance to call yourself anything you like, you can't go with 'John Smith.'"

"Well, what would you suggest?" the Doctor, or was it John, turned toward Rose. They seemed to be ignoring their audience.

After a moment, she replied, "How about Don Noble? You're part Donna now, even if you didn't get the ginge part."

"Oi! Watch it," John, or Don apparently, looked scandalized, "Plus it's only part. John Noble?"

"Don Smith," Rose decided, "Flows better. Rose Smith sounds better then Rose Noble anyhow."

Oblivious to the implication, Don or John or whatever replied, "Why do you get to decide, eh? It's my name."

"Well, pick one. I swear, if you don't, I'll start calling you 'Handy'."

"Oi!"

"Oi!" Tony repeated mockingly, "Whoever you are, Don or John or Doc', you said you knew something we didn't."

"Oh, yes!" the Doctor Donna-John turned toward them again, "Sorry. You're Tony Stark. Iron-man."

"Ooh, let me," Rose cut in, "Thor, the Norse alien god; Captain America, real name Steve Rogers; Natasha Romanov, also known as Black Widow; Bruce Banner, who becomes the Hulk; and Hawkeye, real name Clint... um... Beaton?"

"Barton," her partner corrected, gratefully taking his sandwich from the waitress, "Right on all other accounts. In other words, the original Avengers!"

The two beamed at them, looking thoroughly impressed. Rose took a bite out of her vegetable filled samosa. They now had the Avengers' full attention.

"How did you know that?" Natasha questioned, "The Avengers Initiative was top secret. Even after all this I doubt the press has uncovered that much."

"Is it?" Smith or Noble looked apologetic, "Whoops, a little early then. Must be the Chitauri invasion."

"How we know isn't important," Rose covered, "We can't even interfere until we know a little more about this universe. Brush up on our history, if you will."

"You speak in riddles," Thor proclaimed, "Speak your purpose, if not of yourselves."

"We have a letter to deliver," the enigmatic man said, pulling the letter from his jacket pocket, "and a ride to give. And we sort of can't do it without you."

"Second test drive, this is the only way we'll get it right for sure," Rose added, though no one understood her meaning, "I think I can live with John Noble."

"I'll add a Wilfred in the middle for her gramps," newly-dubbed-John seamlessly replied. He reached over the table awkwardly to hand the letter to Steve.

Steve tore open the letter and read it with an incredulous expression while John dug into his shawarma.

"What is this?" Steve asked, an angry and hurt tone in his voice, "and how is it in my handwriting? I never wrote this."

Ignoring Clint who had exclaimed a flat 'what?' and grabbed the letter, John explained, "It's a letter from your future self. You just created a fixed point."

"Meaning we have to do exactly what the letter says or we will end up creating a paradox," Rose chimed in, "and you really don't want to do that. Universe exploding business, that is."

"You missing a date with a girl could explode the universe?" Clint flatly asked Steve.

"But I can't do that. I'd have to, well, go back in time or something to get there," Steve added angrily.

"Well it's a good thing we happen to have a time machine in need of testing," Rose leaned back in her chair, "otherwise, the universe would be cooked."

"Time machine?" Tony and Bruce asked simultaneously. They glanced at each other before Bruce gave the go-ahead for Tony to say, "but that's completely impossible."

"Humans have no knowledge of Asgardian methods of transport through the Yggdrasil," Thor put in, "I'll admit, I have not heard of such a thing, but surely we cannot claim it to be impossible."

"Weelll, technically the technology doesn't exist in this universe," John elaborated, "We were actually in a different universe, building the TARDIS-"

"That's our ship. Time And Relative Dimension In Space, though I guess we have to add 'universe' into that mix somewhere," Rose continued.

"Alternate slash Relative Dimensions. TAARDIS?" John tried, drawing out the 'a' with an unimpressed look on his face.

"Anyway, we used a shortcut to grow the core, and it ended up being more explosive than we were expecting," Rose looked a bit guilty, "Bam! Here we are, new universe."

"That doesn't explain-" Tony started.

"The point is, that we don't actually know much about this universe yet, and it's impossible for us to pilot through anything but conventional space until we can lock on to at least two fixed points," John interrupted, "Then I can create an algorithm around the relative temporal distance between them."

"So... once you have the algorithm, you can safely travel to any time or space in this universe," Bruce finished hesitantly.

"Exactly," John beamed.

"How'd you get your first point?" Tony asked.

"We were in the future a bit," Rose answered, "Well, I say a bit, a few hundred years really. It's how we knew who you were. Original Avengers, very famous."

"Couldn't do much there, so we just punched in random coordinates and allons-y! Here we are!" John wiped his mouth with a napkin, done his sandwich.

"What's this all have to do with me?" Steve asked. All the talk about relative temporal distances and time and space algorithms went way over his head.

"We landed in Cardiff about six hours ago," Rose explained gently, "And we met a woman whose mother was named Margaret Turner. Maiden name Carter."

Steve could only stare in shock.

"She gave us the letter, and told us where to meet you," Rose finished, "and here we are."

"So how about it," John offered with a grin, "care for a trip?"

* * *

**Taking Steve back in time is a cliche, but a fun one. Incidentally, shawarma is alright but samosas are what it's all about.**


	2. Branching Out

"It's bigger on the inside!" were Tony's first words upon entering the red police box behind John, Rose, and Steve. Clint and Natasha had returned to SHIELD for debriefing along with Thor; and Bruce had stolen Tony's penthouse bed at first sight. Pure exhaustion had won over his curiosity, but nothing could stop Tony Stark from coming aboard a real time machine, even if the letter didn't mention him being there at all. Steve had barely had time to find a period-appropriate dress shirt and jacket.

"Oh, it's been so long since I heard that," Rose sighed, smiling, "Never thought I would again."

"Unfortunately, there wasn't enough material in our universe to create a working chameleon circuit," John provided for explanation from across the console, "so we had to make due with fitting it inside an actual police box."

"What? Why a red police box?"

"None of the blue ones we found were structurally sound enough," Rose told him, grinning. She knew exactly what he meant.

Steve was speechless. He tuned out the time travelers' bickerings about which instruments to use to take readings, and Tony's rapid fire questions about said instruments. Instead, the soldier took in the ship's interior. Pristine white walls were covered in silver orbs, and the floor and roof were made of steel grates. A silver column resembling a tree reached from floor to ceiling, housing a glass cylinder filled with some sort of glowing blue machine. The base and roof of the column weaved delicately like roots through the steel grating, and Steve could see the floor above and the floor below through the grates, giving the room a towering feel. More tree-resembling columns lined the walls, with a leafless branch hanging low by the ordinary police box door being used for coats. For a machine, it looked very organic and alive.

"-just the flux reader and the coordinates are necessary, we need to conserve energy anyway until we can find a power source," John said while wheeling over what looked like a whiteboard. Rose nodded and flipped the appropriate switches while moving around the console.

"How are we going to set the coordinates to get us to London in 1947?" Tony asked while studying the current readouts the screen was giving.

"Easy," Rose told him, brushing back her hair and smirking, "The letter only mentions Cap making one stop."

"Meaning no matter the coordinates we set, we'll end up exactly where he needs to be," John faltered, "probably."

Referencing the letter, John wrote out the exact human time and date they intended to leave down to the millisecond, along with their current coordinates on one side of the board. The former was written in chicken-scratch English, the latter in delicately interwoven circles. He also wrote the human time and date of their estimated arrival, leaving the space below blank.

"What do you mean, 'probably'?" Steve asked.

"There is a teeny tiny itty bitty little chance of, well, imploding and destroying the universe,"

John flipped a switch and started turning a pulley rapidly, while Rose pushed some buttons on the screen and grabbed a wheel to stop it from spinning on its own. Before Tony or Steve could utter a 'wait, what?' the TARDIS gave a great shudder and jerked them to the side. Rose focused on stabilizing their destination while John danced around hitting random buttons and pulling strings for whatever reason. Tony and Steve meanwhile clung to the railings for dear life. Moments later they ground to a shuddering halt.

"H-have we landed?" the billionaire's voice came out at a higher pitch than he would admit later.

"Successfully in 1947 London," Rose declared, checking the screen, "there used to be a sound so you could tell. A sort of wheezing."

The part-Time Lord blushed while copying down the temporal coordinates on the whiteboard, and mumbled something about not installing a parking break yet. Steve ignored him, feeling a great excitement and trepidation as he approached the door. He stepped outside.

Where daylight and bustling New York had been, all steel buildings and wide streets, there was now the pitch black of night time and cobbled narrow roads. The type of night lamps Steve remembered lined the street, and a block down he could see another police box identical to the one he just stepped out of. The entire scene was covered in an inch of snow. He ran a hand through his hair in disbelief.

Steve whirled around when he heard a clatter. Unbelievably, standing not twenty feet from him was Peggy Carter herself, staring at him in equal shock. She had dropped her bag upon seeing him, the contents clattering out. Chivalry won over his shock, and Steve walked forward to help her pick up.

"I believe I still owe you a date," Steve tried, smiling tentatively.

"Y-yes, well," Peggy adjusted herself, "An explanation and perhaps a walk home should do," she eyed his bruised face, and the three strangers peering absurdly out of the police box behind him.

I'm not sure I understand it all myself," Steve admitted, glancing at the trio leaning out of the TARDIS.

"Don't mind us," John called, "we'll need time to work out the formula. Just remember the letter, don't talk about the future, and be back by morning!"

The couple popped back into the ship, a second later followed by Tony who used the time to give Steven an obnoxiously suggestive wink. Peggy shot the billionaire an incredulous look, and Steve couldn't blame her. It looked like his three companions were shacking up for the night in a box with barely the room for one. Chucking, he offered her the bag and his arm.

"Oh, we are brilliant!" John declared to Rose, "How often do we get perfect landings like that? Right place, right time, and not ten metres from the woman we were looking for!"

"1947 London," Tony breathed, "actual time travel. Just set the coordinates and go!"

"That was the easy part," John pulled out a pair of glasses, studying his board, "this, though... it's definitely not like home."

"Anything I can do to help, Doc'?" Tony offered, "I am sort of a genius."

John looked reluctant, but Rose said, "Best give him the explanation you gave me at least. He'd probably understand it more."

John nodded, and flipped the board. He began drawing an elaborate diagram with great speed and precision.

"People assume that time is linear, a strict progression of cause to effect. But from a non-linear, non-subjective viewpoint, it's actually more of a big ball of wibbly wobbly time-y wimey... stuff."

"Some of the Gallifreyan terminology doesn't translate," Rose offered to a laughing Tony.

"Yes, thank you Rose. These points here indicate fixed points in history, and they interweave with the rest of space-time in whichever direction they affect," John continued.

"So actions I take today can affect previous events? Like a retcon?"

"Yes, exactly! Well, sort of, you generally only see those sort when time travel is involved. For instance Captain Rogers reading that letter created a fixed point leading back to the letter written in 1947. The space in-between is all in flux, and the two fixed points lead in opposite directions moving toward each other."

"A self-containing paradox," Tony concluded.

"In this case, yes," John confirmed, "However, that formula doesn't work in this universe, otherwise Rose and I wouldn't need to do this to set the new algorithm."

"How did your Earth come up with this formula?" Tony asked, studying the diagram full of circles and swirls.

"Earth didn't. The founder of the Time Lords of Gallifrey did," John said pompously.

"So in addition to being from a different universe, you're also an alien. Huh."

"Half-alien," Rose chimed in, "but that's a long story. I'm completely human, if anyone's askin'."

John flipped the board over, showing the numbers again. Directly between them he drew what looked like a vertical figure-eight with spirals in opposite directions interweaving symmetrically. He added various dashes and lines along the curves.

"This is the base formula for self-containing paradoxes. You can tell right away that this formula won't work because when the coordinates are written in Gallifreyan they should be mirror images of each other," he paused, tongue to the roof of his mouth while he examined the two sets of writing.

Less precisely, he began to sketch a new diagram underneath in interweaving lines, "Time Lords, and by extension me, would see the time vortex similar to the formula. With these, it would fit together more like..."

John trailed off uncertainly, sketching and erasing. The new diagram had started similarly to the figure-eight above it, but was slowly losing it's various swirls and marks as John straightened out several lines and spiraled others around each other. He looked more uncertain and vaguely confused as he went along, mouth open and tongue touching the roof of it.

After four hour and two more whiteboards, Rose forced the two geniuses to take a break with tea and tuna sandwiches that went horribly together. Tony looked frustrated at his own limits and her Doctor looked absolutely miserable. For the first time in his memory he was looking at time and failing to understand.

"Maybe it's me," John started, "I have the memories, the feelings, but maybe my my mind is less like his than we thought."

"Who?" Tony asked.

"That's not true," Rose ignored the billionaire, "You said it yourself. You can still see how time works the same way, and it's enough to put together this new formula. I doubt that Time Lord founder guy worked out the original in four hours."

"Oh, if Rassilon heard you call him 'Time Lord founder guy'," John giggled with a scandalized look. He felt reassured.

"Besides, you got that far," Rose indicated the newest diagram, "It just looks like a crazy tree to me."

"Tree," Tony repeated, eyes wide, "Hey Doc', I got an idea."

He jumped to his feet, carrying his sandwich with him as he flipped the first board. Erasing the original circular diagram for the space, he grabbed a marker and started drawing.

"Doc', you're lucky I read the files and have a photographic memory. When Thor first landed, he left a big stamp on the Earth. They took some photos of it, and he told us that it was a tree. The Yggdrasil tree he called it," Tony sketched out the diagram in a large circle.

The tree was drawn with groups of lines interwoven with each other, and while some roots and branches ended randomly, others reached the edge of the circle and weaved around to meet their partners above. Tony stepped back, eating the last bites of his sandwich as he checked the finished diagram to the one in his memory.

"Oh," the human Doctor breathed, "Oh, brilliant! Oh, too much stuff in this head, too many old memories, too set in my ways! Oh, Rose Tyler, you're brilliant!"

"Hey, whose perfect memory are we talking about here?" Tony interrupted.

"You! You beautiful thing, look at you, trying to show me but I wouldn't listen," Tony opened his mouth at the strange praise, before realizing it wasn't meant for him.

John whirled around, studying the ship, "Look at her! She formed as we were hurtling into this universe, of course she'd be smart enough to give us a hint," he hugged a pillar which was clearly meant to resemble the trunk of a tree.

Tony fumed as John began redrawing his formula, moving branches and rewriting equations to match the Yggdrasil drawing. Rose giggled and went to pat him on the back.

"Give him a moment and he'll remember," she reassured through her laughter.

John heard her clearly and whirled around, looking sheepish.

"Oh, I'm sorry. So so sorry. Of course you too, you brilliant man. Look at us. Two brilliant humans and a barely acceptable Time Lord, re-inventing one of the oldest algorithms in the universe," he pulled Tony and Rose into a hug, before whirling back to finish his diagram.

"Oh yes! Only four hours! Eat your hearts out, Rassilon!"

**Thanks to my one reviewer and to those of you who decided to follow this catastrophe. Feb break is just starting for me, so I might be able to throw something further together, but if not than she ends here. Thanks for reading!**


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